Precision engineering is a sub-discipline of mechanical engineering concerned with designing machines, fixtures, and other structures that have exceptionally high tolerances, are repeatable, and are stable over time. These approaches have applications in machine tools, micro-electromechanical systems (MEMS), optomechanical designs, and many other fields, such as high-precision coordinate measurement machines (CMM's). In precision Engineering components are getting smaller and tolerance becomes tighter, so demands for accuracy are increasing.
A coordinate measuring machine (CMM) is a device for measuring the physical geometrical characteristics of an object. This machine may be manually controlled by an operator or it may be computer controlled. Measurements are defined by a probe attached to the z-axis of this machine (however in other embodiments it may be attached to other axes as well). This probe contacts the part of interest and allows collecting discrete points on the object's surface.
The typical CMM is composed of three axes, X, Y and Z. These axes are orthogonal to each other in a typical three dimensional coordinate system. Each axis has a very accurate scale system that indicates the location of that axis. All three axes are displayed on a digital readout. The probe is used to touch different spots on the object being measured to obtain probe points. The machine then uses the X,Y,Z coordinates of each of these probe points to determine the size and position of the object. There are newer models that have probes that drag along the surface of the part taking points at specified intervals. This method of CMM inspection is less accurate than the conventional touch-probe method but in most cases it is faster. The next generation of scanning, known as laser scanning, is advancing very quickly. This method uses laser beams that are projected against the surface of the part. Many thousands of points can then be taken and used to not only check size and position, but to create a 3D image of the part as well. This “point-cloud data” can then be transferred to CAD software to create a working 3D model of the part. The laser scanner is often used to facilitate the “reverse engineering” process. This is the process of taking an existing part, measuring it to determine its size, and creating engineering drawings from these measurements. This is most often necessary in cases where engineering drawings may no longer exist or are unavailable for the particular part that needs replacement.
A coordinate measuring machine (CMM) is also a device used in manufacturing and assembly processes to test a part or assembly against the design intent. By precisely recording the X, Y, and Z coordinates of the target, points are generated which can then be analyzed via regression algorithms for the construction of features. These points are collected by using a probe that is positioned manually by an operator or automatically via Direct Computer Control (DCC).
One of such CMM's is known from: M. M. P. A. Vermeulen, et. al., “Design of a High-Precision 3D-Coordinate Measuring Machine”, Annals of the CIRP Vol. 47/1/1998, p. 447-450. The known CMM comprises a movable platform to which a probe has been connected. The probe is mounted to on a vertical slide in the movable platform such that it can be moved up and down in a z-direction. The vertical slide is provided with an internal scale. The movable platform is movable in the x-direction and y-direction through two orthogonal beams. This is achieved in that the movable platform is coupled to a base through two intermediate bodies which are movable along respective orthogonal guiding beams on the base. The intermediate bodies are provided with respective holes through which the orthogonal beams can be moved. The x-scale and y-scale are supported on the respective orthogonal beams, while the respective measuring heads are provided on the intermediate bodies. The machine slides (beams and slides) are built up as a closed-box plate construction. For all slides passively compensated thrust air bearings were chosen.
A problem with the known CMM is that during movements of the platform relatively large masses are moved.